This invention relates generally to package wrapping machines utilizing thin, stretchable film to wrap and display articles contained in trays and, more particularly, to an improved control system for selecting one of at least two film widths and for controlling the handling of the film dependent upon the film width selected. Package wrapping is thus individualized for each available film width to improve film tension control for packages wrapped by film wrapping machines incorporating the present invention.
A variety of film wrapping machines are known in the prior art. Two examples of such machines are illustrated in U.S. Pat. Nos. 3,662,513 and No. 3,967,433. The wrapping machine illustrated in U.S. Pat. No. 3,662,513, relies upon a horizontal folding matrix plate to control the film tension. The folding matrix plate has a passage opening therethrough corresponding in shape to the item to be wrapped. A film sheet is drawn under the matrix plate and thrust bars hold the film against the matrix plate while a package to be wrapped is elevated through the opening to engage and stretch the film. The combination of the matrix plate, the thrust bars and the elevator stretch the film sheet over the package prior to underfolding the film beneath the package to complete the wrapping.
The film wrapping machine illustrated in U.S. Pat. No. 3,967,433, provides for prestretching a film sheet and then elevating a package to be wrapped into the sheet with the sheet then being folded under the package. In order to maintain a predetermined amount of stretch or tension in the film on wrapped packages, the film stretching provided by the prestretching mechanism is simultaneously reduced as the package is elevated into the prestretched film and the film is folded about the package. While neither of these prior art film wrapping machine patents discloses the selection of one of two differing width films, the machines disclosed are typical of stretch film wrapping machines available in the prior art and illustrate two approaches to film tension control in wrapped packages.
A third film wrapping machine which is commercially available appears to incorporate portions of both of the two cited patents. This third machine further provides for automatic selection of one of two differing width films and the selection of the length of the selected film in accordance with characteristics of a package to be wrapped. Film is initially pulled into the machine and at least partially prestretched by a film gripper which grips the end of a continuous roll of film and pulls a film sheet of a selected length. Side clamps then engage the opposite sides of the film sheet and prestretch it prior to the elevation of a package into the film. The film is then underfolded beneath the package to complete the wrapping.
While this third wrapping machine is an advance over the machines illustrated in the two cited patents, manual adjustments are required to obtain high quality wrapping for the various widths and lengths of film selectable by the machine. Control of the opening and closing of film gripping and stretching mechanisms of the wrapper are controlled by air pressure cylinders which are inherently imprecise and variable over time. Adjustments of the air pressure are to be manually made by the operator to improve the wrapping characteristics for varying sized packages and, hence, varying sized film sections. Where such adjustments are not made due to inexperience, inadvertence or slothfulness, the same degree of drawing and stretching is applied to all film sizes with a resulting compromise in the quality of wrapping.
It is, thus, apparent that the need exists for an improved control system for providing individualized wrapping parameters for each film size when at least two film sizes are selectable by a film wrapping machine.